The present invention relates to fuel cells; more particularly, to stacks comprising a plurality of individual cells being both physically separated and electrically connected by interconnect elements; and most particularly, to such a fuel cell stack wherein the interconnect elements are thin foils and the spacers are laminates of foils formed alternately of superalloy and a compliant material.
Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by a permeable electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a xe2x80x9csolid oxide fuel cellxe2x80x9d (SOFC). Either pure hydrogen or reformate is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. Each O2 molecule is split and reduced to two Oxe2x88x922 ions at the cathode/electrolyte interface. The oxygen ions diffuse through the electrolyte and combine at the anode/electrolyte interface with four hydrogen ions to form two molecules of water. The anode and the cathode are connected externally through the load to complete the circuit whereby four electrons are transferred from the anode to the cathode. When hydrogen is derived from xe2x80x9creformedxe2x80x9d hydrocarbons, the xe2x80x9creformatexe2x80x9d gas includes CO which is converted to CO2 at the anode/electrolyte interface. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.
A single cell is capable of generating a relatively small voltage and wattage, typically about 0.7 volts and less than about 2 watts per cm2 of active area. Therefore, in practice it is usual to stack together in electrical series a plurality of cells. Because each anode and cathode must have a free space for passage of gas over its surface, the cells are separated by perimeter spacers which are vented to permit flow of gas to the anodes and cathodes as desired but which form seals on their axial surfaces to prevent gas leakage from the sides of the stack. Adjacent cells are connected electrically by xe2x80x9cinterconnectxe2x80x9d elements in the stack, the outer surfaces of the anodes and cathodes being electrically connected to their respective interconnects by electrical contacts disposed within the gas-flow space, typically by a metallic foam or a metallic mesh which is readily gas-permeable or by conductive filaments. The outermost, or end, interconnects of the stack define electrical terminals, or xe2x80x9ccurrent collectors,xe2x80x9d connected across a load.
In the prior art, the interconnect elements are relatively thick, flat plates formed of a superalloy or stainless steel. Also, the perimeter spacers that form the gas flow spaces adjacent to the electrodes are typically formed from sheet stock having a thickness selected to yield a desired height of the flow space.
One problem encountered in prior art fuel cell stacks is that they are relatively bulky and heavy. It is very desirable to reduce the height and weight of a stack without sacrificing performance.
Another problem encountered in some prior art fuel cell stacks involves the brittleness of the ceramic oxide electrolyte elements. In some fuel cells, the anode is a relatively thick structural element supporting a thin electrolyte layer and a thin cathode layer. Such a fuel cell is said to be xe2x80x9canode-supported.xe2x80x9d The ceramic oxide electrolyte elements, which extend to the edges of the stack in contact with the anodes, typically are not optically flat and are also quite brittle. The anodes may also not be optically flat. Prior art perimeter spacers, being monolithic, cannot twist to accommodate non-planarities in the electrolyte elements and anodes, so that sealing between the non-flat surfaces becomes difficult. Also, because of the non-flat surfaces, an electrolyte element may be cracked during assembly of the stack. In either case, failure of the stack can occur. Avoiding these problems by finishing the electrolyte elements to be optically flat is cost-prohibitive.
It is a principal object of the present invention to provide a fuel cell stack that is lighter and smaller than prior art fuel cells of the same electrical capacity.
It is a further object of the present invention to provide spacer means for a fuel cell stack that can sealably conform to non-planarities in the electrolyte elements and will not induce torsional stress in such elements.
Briefly described, the interconnects and perimeter spacers for a fuel cell stack are provided as flexible elements which can conform to non-planarities in a stack""s elements. The interconnects are foil elements about 0.005 inches thick, formed of a super alloy, such as HASTELLOY or HAYNES 230, or stainless steel. The thick perimeter spacers comprise a plurality of thin spacer elements. Each spacer element is a laminate of a superalloy and a compliant soft material such as copper, nickel, or mica. The spacer elements can slide past one another; thus the perimeter spacers can be physically thick, to form gas flow spaces within the stack, while also being torsionally flexible.